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Ease the squeeze

London Underground is gearing up for a tunnelling job at Victoria Station requiring technical finesse and immaculate logistical planning.

Heathrow Airport handles about 70M passengers a year and next year will see it opening one of the most vaunted construction projects of recent times, the colossal Terminal 5, to help ease congestion. Much has been made of the project's technical and logistical complexity.

However, a transport infrastructure upgrade project to handle as many or more people, built alongside milling central London commuters, could be reckoned to add extra difficulty.

"Victoria Station is one of London Underground's busiest. It handles 80M passengers a year," says London Underground project director and civil engineer Peter Lynch. "We're regularly forced to hold passengers outside the station until there is enough space to get people moving again." Closures last a few minutes at a time. Forecast increases in demand of 20% by 2016 will worsen the situation.

The £509M upgrade is a congestion-busting project. It will also greatly improve emergency evacuation and access for the emergency services. Work falls inside Transport for London's (TfL) investment programme and is separate from public private partnership (PPP) work to upgrade the underground system. LU is directly managing the project and is rolling planned PPP station modernisation works into the station upgrade contract.

Victoria Underground Station has been under pressure since the Victoria Line was tunnelled beneath the 19th century Circle and District Lines in the 1960s.

There are two ticket halls at Victoria, one under the bus station in front of the mainline railway station, and one under Victoria Station House. One serves the Circle and District Lines and the other the Victoria Line.

The upgrade will enlarge the existing Victoria Line ticket hall and add a new ticket hall providing access from the north side of traffic-choked Victoria Street. The north ticket hall will add a new access to the Victoria Line, using platform space on the less-busy northern end of the platforms. New tunnels will link to the District and Circle Lines and the south ticket halls.

Superimposing the London street map on the new underground layout, it is immediately clear how sensitive the project will be. To build the ticket halls, large excavations will have to be opened up in two of the capital's major thoroughfares – Bressenden Place and Wilton Road.

Tunnels will thread over and under the Victoria Line platform tunnels, under the masonry cut-and-cover tunnels of the District and Circle Line, and beneath foundations of existing buildings.

"We're talking about a major intervention in a very congested urban location, but we have to keep the station open, traffic moving and people flowing," Lynch says. When he says it will take "careful construction planning, detailed traffic management and effective consultation with local stakeholders", he is verging on understatement.

The north ticket hall will be built under Bressenden Place, a four-lane, one-way racetrack for the armies of vehicles that pound through London SW1 each day. "To keep traffic moving we'll use top down construction. We'll shift traffic over to one side of Bressenden Place and put in secant piled walls and a lid on the other. Then we'll swap sides," says Mott MacDonald project manager Mark Leggett.

Mott MacDonald is lead consultant for all civil, mechanical and electrical, environmental, traffic and utility works. Architecture is led by Weston Williamson. London Bridge Associates – what used to be Taylor Woodrow's tunnelling division – is embedded in the design team to advise on buildability and construction programming. Other team members include transport planning specialist MVA, noise and vibration consultant Rupert Taylor, and quantity surveyors Corderoy and Franklin & Andrews.

Secant piled walls for the north ticket hall will be three storeys deep, but Leggett reassures "the box is large but conventional". The challenge is one of traffic management and construction logistics.

For expansion of the south ticket hall, Wilton Road will have to be closed for a short period to allow a similar secant piled wall and roof slab to be constructed. Here there will be the added challenge of excavating close to the listed Victoria Railway Station and adjacent to the Apollo Theatre.

Away from the traffic, tunnelling will face different and more technical challenges. Pedestrian tunnels will be about 5m diameter, capable of conveying crowds of commuters and sized for the long-term.

The geometry of the tunnels has been dictated by the need to navigate around existing tunnels, foundations and services, and by the desire to keep passenger journey times as short as possible.

"The deeper you go the longer the journey time is through the station," Lynch says. "Deep tunnels would rather defeat our objective of minimising passenger journey times."At Victoria, tunnelling will be relatively shallow at the interface between London Clay and overlying terrace gravels. Ground treatment will be needed to make the gravels self-supporting and to deal with water. "It is possible that ground freezing may be used, but more likely it'll be a combination of jet grouting from the surface and from tunnel level," Leggett says. "Ground freezing can potentially impact on the construction timetable – achieving a freeze can be a slow process." Grouting trials are being planned and will be carried out next year in order to refine the methodology.

Tunnelling itself will probably be carried out using a mining excavator or small road header, Leggett says. Hand excavation will be required in places. Mott MacDonald is proposing that canopy tubes be installed ahead of tunnelling to provide temporary support in addition to the jet grouting. Forward probing will be used to detect water and zones of poorly treated ground.

Tunnel linings are likely to be sprayed concrete, "although there may be contractors out there who'll opt for segmental cast iron in places". Sprayed concrete will enable the contractor to negotiate the frequent changes in direction and create the numerous junctions more easily, Leggett says.

Allowable ground movement will be extremely small. Buildings and infrastructure will be instrumented. "We'll create a data bank on the positions of above and below-ground structures before the main works start," Leggett says.

Movement below the District and Circle Line tunnels is limited to +/-4mm. Prior to construction of a new escalator tunnel, threading through the gap beneath the District and Circle Line and above the Victoria Line, the District and Circle Line tunnels will need to be underpinned. This will be carried out during engineering hours.

Grout injection will be used to compensate for heave or subsidence picked up by the real-time structural monitoring system.

But the tunnellers' biggest challenge will be neither buildings nor tunnels but the existing escalators. "They're very sensitive to movement. To compensate for slight ground deformations, a real-time monitoring and jacking system may be installed during the works," Lynch says.

LU is submitting a planning application this month and anticipates a public inquiry in June next year. Subject to approval the main works contract would be awarded in mid-2009, with construction starting soon afterwards." says Lynch.

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